Bridge mode

2. Bridge mode

In Bridge mode, a packet is transmitted to the radio channel
immediately, without any checking whether the radio channel is occupied or
not. If other radio was transmitting simultaneously, a collision would
occur and both packets would be lost. Consequently Bridge mode can be used
only for applications which never generate more than a single message at a
time, e.g. master-slave polling applications. Still appropriate measures
have to be taken to avoid collisions in special situations.

2.1. Bridge mode with Repeaters

Repeaters can be used in the Bridge mode in order to extend the
radio coverage. Considering the repeated packets, it is necessary to
schedule the access to the radio channel to avoid systematic collisions.
In a polling-type network, there is a request packet from centre to
remote, to which the remote responds immediately. When a remote receives
the request directly from the centre, its immediate response would
collide with the repeated request, so it would be never received by the
centre – a perfect example of a systematic collision.

Packet header contains information about the number of Repeaters
on the route, i.e. how many times the packet can be possibly repeated.
This number is decremented when passing through each Repeater. The
remote radio modem which receives the packet must delay its own
transmission for a period. This delay is calculated from the number of
the remaining repetitions, the packet length and the modulation rate in
the radio channel. Repeaters always transmit immediately, without any
delay.

Example:

There are 4 radios in the network operated in the Bridge mode.
Everyone can receive each other except Radio 4, which is not able to
receive Radio 1 and vice versa. Therefore, in the Radio 3 the Repeater
function is turned on, and it mediates the connection between 1 and
4.

First, packet A is broadcast from Radio 1.

Radio 2 receives Packet A and sends it to its COM. In the instant
when it starts the reception of Packet A, Radio 2 calculates (from
information in the received packet header and from number of repeaters
in its own setting) the time delay which is needed for the delivery of
Packet A through the repeater (repeaters). When the response from the
connected device arrives via COM (Packet B), the Radio 2 postpones its
transmission for the delay.

In the meantime, Radio 3 (Repeater) receives Packet A and repeats
it to the radio channel immediately. Radio 4 receives the Packet A and
then Packet B and sends them both to the COM. Packet B is also received
by Radio 3 and immediately repeated. Whenever a radio receives a copy±
of the same packet during the calculated delay, it discards it as a
repeated packet. Note that the picture does not show all the packets at
all the radios.

Repeater is configured in the Settings / Device / Operating Mode
menu, for Radio 3 (left) and Radio 1, 2, 4 (right):

The delay period based on number of repeaters solves the collision
between a repeated packet and a possible response. When more than one
repeater is used in a Bridge-mode network, collisions between repeated
packets from different repeaters may occur. These cannot be solved by
simple delays, rather a sophisticated anti-collision protocol is
required. The RipEX Router mode is recommended to be used in more
complex networks with multiple repeaters. Nevertheless if certain
conditions on signal coverage (sometimes non-coverage) among repeaters,
centre and remotes are met, the Bridge mode for a polling-type
application can be used. See the chapter Bridge
mode in RipEX Manual.

2.2. Time division of responses in Bridge mode

There is also the Tx delay setting in the menu. It shall be used
in Bridge mode if multiple RTUs connected to slave stations reply to a
broadcast query from the centre. It is necessary to spread out their
replies to the radio channel in terms of time, otherwise a massive
collision occurs. It can be achieved by setting the TX delay parameter
to an adequate sequence of TX delays (e.g. 0, 100, 200 ms as in the
example below) in individual remote RipEXes. The slave RipEXes will
enter the radio channel successively and no collisions will
occur.

Note: The TX delay applies to
every packet that is sent out to the radio channel.

Example:

The Centre broadcasts request and the RTUs 1, 2 and 3 generate the
response and send it out to their respective RipEX.

Radios 1, 2 and 3 have the TX delay parameter set to 0, 100 and
200 ms, respectively. Therefore, Radio 1 starts transmitting just after
reception of the frame from COM port. Upon 100 ms later, when Radio 1
has completed transmission, Radio2 starts transmitting. Finally, 200 ms
after the reception of the packet from RTU, Radio 3 starts its
transmission. All three responses are thus sequentially sent to the
Centre and no collision happens.

The TX delay parameter coresponds to multiples of maximum packet
length expected and shall be set in miliseconds. The packet transmission
time through radio channel can be calculated as follows:

t = (n + 12). 8/(b . fec)

where:

t [ms]

– time needed for the packet transmission

n [ - ]

– number of bytes transmitted (consider the longest
possible reply from RTU)